This invention relates generally to temperature stabilizing devices, and more particularly, to temperature stabilizers including positive-temperature-coefficient (PTC) devices for obtaining a constant temperature environment for electronic components. Most electronic components are relatively insensitive to temperature variations, but some components are extremely sensitive to temperature. In many instances, one of the most critical tasks of a designer of electronic systems is to ensure that the environmental temperature is controlled with sufficient precision to accomodate the temperature-sensitive components in a system.
A device that is sometimes used for temperature control is the positive-temperature-coefficient (PTC) thermistor. When a constant voltage is applied to a PTC thermistor, it heats up until a switching temperature is reached, at which point the electrical resistance of the device begins to increase rapidily with temperature. Thus, the power dissipated, which is inversely proportional to the resistance at constant voltage, decreases rapidly with increasing temperature. An equilibrium temperature is quickly attained, at which the power dissipated is equal to the thermal power leaking to the environment surrounding the device.
Early attempts to control the environmental temperature for electronic components have suggested the use of insulation around the entire electronic package. This is not only a bulky solution, but may also consume relatively large amounts of power in the thermistor, since there will be a large amount of thermal leakage from the thermistor to the outside environment. Moreover, this leakage will result in a relatively long warm-up period before the thermistor reaches its equilibrium temperature.
It has been recognized that it would be advantageous to provide some form of insulation between the thermistor and the package on which it is mounted, to reduce the thermal leakage through the path of least thermal resistance. For example, U.S. Pat. No. 3,158,821 to P. G. Sulzer reaches this conclusion, but does not suggest any specific structure for increasing the thermal resistance of the leakage path. A similar conclusion was reached by B. R. Potter et al. in "Temperature Regulation of LiNbO.sub.3 Devices," published in the Proceedings of the 1975 IEEE Ultrasonics Symposium, pp. 499-502. No specific solution was offered, however, and the paper focuses instead on the use of insulation surrounding the entire package.
One important application of temperature stabilization is in conjunction with surface acoustic wave (SAW) devices and related devices that employ shallow bulk acoustic waves (SBAW). These devices operate at frequencies from about 50 megahertz (MHz) to about 2 gigahertz (GHz), and are used as oscillators, resonators, filters, and related components. SAW devices employ metallic electrodes on the surface of a piezoelectric substrate, which is commonly quartz. Quartz has the requisite electro-acoustical properties for SAW devices, and has the additional advantage of being relatively stable over a wide temperature range. Other substrate materials, such as lithium niobate (LiNbO.sub.3), have electro-acoustical properties that are far superior to those of quartz, and may therefore be made to operate at higher frequencies and with lower losses. However, the temperature stability of lithium niobate is many times worse than that of quartz, so the use of lithium niobate must frequently be ruled out only because of its temperature stability characteristics.
It will be appreciated from the foregoing that there is still a need for a simple temperature stabilization device that will be effective to control the operating environment of selected electronic components, such as SAW devices, without the dissipation of large amounts of power. Ideally, a temperature stabilizer should be small enough to fit inside hybrid packages, should be relatively inexpensive, and should be constructed from materials that will not contaminate sensitive electronic components in the package. The present invention satisfies this need.